Standard library in GCC toolchains doesn't contain system calls which possibly depend on hardware layer.Enable -specs=nosys.specs to link stub library or write stub functions by yourself.cf. https://stackoverflow.com/questions/22660011/undefined-reference-to-kill-getpid-and-sbrkhttps://sourceware.org/newlib/libc.html#SyscallsYou may see warnings building nosys library since it is stub.You should write functions if you don't want to see the warnings.